Claims that Australia exported a virus that may have caused US bee colonies to collapse have been questioned with the release of new scientific findings.

US Department of Agriculture (USDA) spokesmen Dr Jay Evans and Dr Yanping Chen question the role of the bee virus in the so-called Colony Collapse Disorder (CCD).

Their study has found the suspect virus was already in the country before Australian imports started.

"[The virus] was present in US bees collected several years prior to CCD, and prior to the recent importation into the US of honey bees from Australia," the researchers write in the American Bee Journal.

The $US17 billion ($18.8 billion) US honey bee industry has been thrown into disarray since a mysterious agent caused hives to collapse.

During the 2006-2007 northern hemisphere winter, a quarter of beekeepers lost more than half of their hives, jeopardising the livelihood of farmers and orchardists who need bees for pollination.

Earlier this year a study in the journal Science fingered Australian bees as a possible culprit.

Researchers identified Israeli Acute Paralysis Virus (IAPV) in imported Australian bees, a virus that had been regularly turning up in hives affected by CCD.

There was talk of the USDA banning imports of Australian bees, putting at risk the country's $5 million live export market, and orders from Australia have now been cancelled.

But Dr Evans and Dr Chen have tested samples from commercial US beekeepers collected between 2002 and 2007 and found the virus has been in the US since at least 2002.

Australian bee expert Dr Doug Somerville of New South Wales Department of Primary Industries says Australian packages only arrived on the US shore in 2005, so the link with Australia "is just garbage".

Different strains of virus

But the USDA researchers are not letting Australian bees completely off the hook.

They say their study found there were different strains of IAPV and it is possible that Australian strains are more virulent in US bees than the strains that were there before.

Dr Evans and Dr Chen now want to study Australian strains to compare these to the strains that were present in US colonies that collapsed.

Dr Evans does say there is a "less than 50 per cent chance" that the problem strain is from Australia.

"The burden of proof in terms of imports now seems to be to show that Australian strains, per se, are more strongly connected with CCD than are strains already in the US," he said.

Dr Somerville thinks the role of IAPV is "academic".

"Bees are suffering a whole range of other issues which are probably far more serious," he said.

He says viruses only tend to be a problem for bees when they are already sick due to pests or diseases, malnourished or suffering from environmental factors.

Dr Somerville says US bees have been under more pressure than Australian bees due to factors such as the parasitic varroa destructor mite and exposure to agricultural chemicals.

He says Australian bees tend to feed on native flora rather than the pesticide-sprayed weeds and crops of US bees.

"Once an animal is stressed for a range of reasons, then your viruses come into play in terms of having an impact," he said.

"A number of people believe that's what's happening with CCD in America."